1. Field of the Invention
The present invention relates to a semiconductor integrated circuit, and in particular, to a semiconductor integrated circuit into the storage circuit of which data can be written.
2. Description of the Prior Art
It has been known that a very small amount of a radioactive material contained in a package or radioactive rays radiated from solder when a chip is mounted on a substrate cause a nondestructive malfunction, so-called “soft error” in a semiconductor integrated circuit. It has also been known that cosmic rays, in particular, neutron beams cause the same malfunction in a semiconductor integrated circuit. A soft error in a semiconductor element can cause failure of an entire system. For this reason, in a system requiring high reliability, the use of elements highly resistive to a soft error and circuit design are implemented.
Error Correcting Code (ECC) has been proposed to protect circuitry in a memory element from a soft error. In ECC, parity data into which some data are put together is stored in another region when data is stored in a memory element to compare the data stored in the memory and the parity data, thereby detecting an error. If an error is detected, anew data is again overwritten. The ECC is effective for the memory element, but it increases the overhead of a circuit when applied to a small-capacity storage circuit such as a latch circuit in logic circuits.
As another method, there has been a device in which the same data is stored in a plurality of latch circuits and the stored data is compared when read to take a majority decision, thereby transmits correct data even if the stored data is destroyed by the soft error (refer to, for example, Japanese Patent Laid-Open No. 63-49860). A method in which a majority decision is taken requires three or more latch circuits to increase the circuit scale. For this reason, the conventional measure for preventing circuits from malfunctioning due to the soft error causes a problem of increasing the circuit scale.